The use of fossil fuels in combustion installations (furnaces, boilers and the like) results in the production of sulfur-containing compounds, in particular, sulfur dioxide, which must be removed from the exhaust gases prior to release into the atmosphere. Various particulate additives are known, including calcium carbonate, magnesium carbonate, limestone, dolomite and calcium hydroxide, which react with sulfur dioxide in a manner which causes them to be sorbed onto these additives. The particulates then may be removed from the exhaust gases by various means, such as by filtration of the flue gas or electrostatic precipitation, and then discarded or recycled. The efficiency of such a sulfur scrubbing process depends in part upon the efficiency of contacting the particulate additives with the exhaust gases from the combustion installation. However, attempts to maximize efficiency have been complicated by the fact that reaction of the additives with sulfur dioxide occurs within a particular temperature range which is, in the case of calcium carbonate, about 2000.degree. to 1000.degree. F. Since the temperature within the combustion zone of a furnace is well above this temperature, i.e., usually 2600.degree. F. and higher, the contact between the particulate additive (sorbent) and the gaseous combustion products must take place outside of the combustion zone to avoid sorbent deactivation. If the dry particulate additive is at any time exposed to a temperature above about 2000.degree. F., significant sorbent deactivation occurs rendering the sorbent chemically useless for the desired sorption reaction. However, if the sorbent is added at a location in the furnace where the temperature is too low, the desired reaction will not readily occur.
Therefore, there is in the art a problem of achieving optimum dispersion of the particulate additive within the combustion gas stream, contacting the particulate additive with the gas stream at the favorable temperatures (2000.degree.-1600.degree. F.), while minimizing or eliminating exposure of the particulates to a temperature above about 2200.degree. F.
Various methods are known to attempts to deal with this problem. According to one known method, the additives are introduced in a dry form into the combustion region above the combustion zone with the aid of air jets. See U.S. Pat. No. 110 -345 However, this subjects the solid additives to the entire temperature spectrum of the flame and furnace, which leads to deactivation of a substantial portion of the additive.
According to another known method, the solid additives are premixed with the fuel (such as coal) before the fuel is introduced into the combustion region. See U.S. Pat. No. 3,746,498. Since the fuel is introduced directly into the combustion zone, this subjects the additives to the extreme temperatures within the combustion zone, thereby leading to deactivation and loss of additive activity.
According to a third known method, the dry additives are injected with the secondary air into the burner around the combustion zone. See U.S. Pat. No. 4,331,638. Again, this leads to substantial deactivation of the additive.
According to a fourth known method, the dry additive is injected below the burner zone (combustion zone), which leads again to sorbent deactivation due to the high temperature. See U.S. Pat. No. 4,440,100.
In U.S. Pat. No. 4,555,996, commonly assigned a process is described for reducing the gaseous sulfurcontaining products in gases resulting from combustion of fossil fuels by injecting chemical additives as aqueous dispersion into the combustor in a zone which is outside of the combustion region, but which is at a temperature exceeding about 2200.degree. F. The spray composition is injected at such a location, velocity and liquid droplet size that it remains substantially aqueous as it disperses within the combustion gases and thereafter flows with the combustion gases, while water in the particles evaporates, into a second zone having an upper temperature limit of about 2200.degree. F. A substantial portion of the spray composition is thereby converted to solid particulate matter which, within the second zone, is capable of reacting with the sulfur-containing gases.
The present invention is an improvement upon the process described in U.S. Pat. No. 4,555,996.
Therefore, is it an object of the present invention to provide an improved method for reducing the gaseous sulfur-containing products in gases resulting from combustion of fossil fuels.
It is another object of the present invention to provide a method for reducing the gaseous nitrogencontaining products in the gases resulting from combustion of fossil fuels.
These and other objects and advantages of the present invention will be apparent from the following specification, accompanying drawing and appended claims.